Tungsten alloy products

ABSTRACT

Dispersion alloy products of tungsten are disclosed containing grain-growth promoting additives uniformly distributed in the tungsten matrix along with a concentration gradient of thoria particles. The thoria particles are distributed in the tungsten matrix so as to provide a compositional gradient which diminishes continuously with increasing distance from the exterior surface of the product to the product interior. In certain cases upon recrystallization of a wire product fabricated with the dispersion alloy material, there is also formed a central core of relatively large tungsten crystals and a shell of smaller more equiaxed crystals near the exterior surface portion of the particular product. In other embodiments, large interlocked grains were formed throughout the wire. Methods of producing these dispersion alloy products are also disclosed.

States Patent [191 l t I l l Dunham Apr. 9, 1974 [54] TUNGSTEN ALLOYPRODUCTS Primary I%-aminer-(i;1rl{D. Quarfor'ih Assistant .\'amin'er unt[75 Inventor: Th9mas P Cleveland Attorney, Agent, or Firm-John F.McDevitt; Henry P.

Helghts Ohm Truesdell; Frank Y. Neuhauser [73] Assignee: GeneralElectric Company,

Schenectady, NY. ABSTRACT [22] may May 1 1972 Dispersion alloy productsof tungsten are disclosed containing grain-growth promoting additivesuni- [21] Appl. No.: 248333 formly distributed in the tungsten matrixalong with a concentration gradient of thoria particles. The thoria [52]Us Cl 29/1825, 29/182 75/207 particles are distributed in the tungstenmatrix so as to 75/206 provide a compositional gradient which diminishes[51] Int. Cl B27! 1/00 Pntinuwsly with Increasing d'stance m time [58]Field of Search H 29/182. 82.5; 75/207 nor surface of the product to theproduct interior. in 75/206 certain cases upon recrystallization of awire product fabricated with the dispersion alloy material, there is[56] References Cited also former:i a cgnltlralfcorelof relativelylargedtungsteln crystas an a s e o sma er more e ulaxe cr sta s UNITEDSTATES PATENTS near the exterior surface portion of the particular1,082,933 product In other e nbodirnents large interlocked 75/207 grainswere formed throughout the wire. Methods of producing these dispersionalloy products are also disclosed.

18 Claims, 3 Drawing Figures t 1 l l l l I l Pm: rear/01v 015 mm:(M/CPONS) WIPL 9X15 1 TUNGSTEN ALLOY PRODUCTS BACKGROUND OF THEINVENTION Thoriated tungsten materials commonly are used in twodifferent types of applications, namely, as lamp filaments in shock andvibration resistant lamps and as electron sources in such items as powertubes, discharge lamps and welding electrodes. While the known materialsperform adequately in some of the above applications, there areshortcomings'associated with them so that some need for an improvedmaterial exists in both areas of use. For the product application oflamp filaments, the conventional thoriated tungsten materials having auniform distribution of thoria particles throughout the tungsten matrixare effective in increasing the recrystallization temperature oftungsten and, in addition, offer some restraint to grain boundary motionat lamp-operating temperatures thereby stabilizing a small grain size.Unfortunately, in obtaining a higher recrystallization temperature fromthe thoria addition, there is a drawback in that the resulting smalltungsten grain structure is not creep resistant andis subject to lampfilament sag and other maladies common to small grain size materials.While some lamp designs can tolerate the low creep resistance of theconventional thoriated tungsten material in order to obtain the benefitsof enhanced shock resistance and vibration-damping characteristics,there is still a distinct need for a creep resistant wire having a highrecrystallization temperature for use in high efficiency rough serviceelectrical lamps.

The electronic applications mentioned above are predicated on theinherent instability of Th: in tungsten at elevated temperatures. Whenthoria reduction occurs, thorium atoms diffuse to the surface andeffectively lower the work function of the surface thereby providing anefficient electron source. Electronic applications which put the mostsevere demands on thoriated tungsten materials are those which utilizesmall diameter wires as grids and electron sources in high powerrectification tubes. In these applications, dimensional stability of thecoiled or wound part is required at extremely elevated temperaturesabove the recrystallization temperature of tungsten. Significantproblems encountered with such utilization of the conventional thoriatedtungsten materials are difficulty in fabricating the small diameter wireneeded and poor creep resistance above the recrystallizationtemperature.

It would be desirable in both of the foregoing product applications,therefore, to provide a dispersion alloy product of tungsten havinggreater structural stability at elevated temperatures than the knownmaterials.

SUMMARY OF THE INVENTION It has now been discovered that a dispersionalloy product of tungsten containing grain-growthpromoting additives asdisclosed in US. Pat. No. 1,410,499 and thoria with the thoriaconcentration being varied in accordance with liquid diffusionprinciples hereinafter defined provides a novel material which is botheasier to fabricate into wire and other forms than is the conventionalthoriated tungsten material and which further exhibits greater stabilityand creep resistance upon recrystallization than said conventional W-ThOmaterials. More particularly, a dispersion alloy product of tungstencontaining the graingrowth-promoting additives and thoria with themaximum concentration of thoria being located at the exterior surface ofsaid dispersion alloy product and with said thoria concentrationcontinuously diminishing with increasing distance to the interiorportion of the dispersion alloy product provides all of the foregoingdesired advantages.

In accordance with one aspect of the invention, a liquid diffusionprocess is employed to impregnate a porous compact of tungsten particlescontaining a uniform distribution of the grain-growth-promotingadditives. By way of example, the tungsten powder compact was preparedby treating blue tungsten oxide powder (approximately W0 with aqueoussolutions of potassium silicate and aluminum chloride and then reducingthe chemically treated oxide to metallic tungsten by heating inhydrogen. The pressed compact of tungsten particles containing thegrain-growth-promoting additives and having an interconnected porestructure was first presintered at l,200C in hydrogen and then soaked inwater until all pores were essentially filled. The water-filled compactwas then immersed in a water solution of thorium nitrate for varioustime periods depending uponthe thickness of the thoriated tungsten alloyshell desired in the final alloy product. Removal of the solvent wasaccomplished by drying the impregnated compact which was then processedby conventional presintering and sintering operations. The end result isa sintered material having a continuously diminishing concentrationgradient of thoria in the direction of the central core of the porouscompact. The maximum thoria concentration is produced at theexterior-most surface portion of the porous compact with saidconcentration continuously diminishing as the distance increases to thecenter core of the porous compact in accordance with operatingprinciples of liquid diffusion. Upon recrystallization of wires producedby swaging and wire drawing, different metallurgical structures could beobtained depending on the thermal treatment and the specific ThOgradient present in the wire.

In a preferred embodiment of the present invention, the thoriaconcentration uniformly decreases from the surface to a very small valuenear the central axis so that a portion of the interior of the ingotcontains essentially no ThO Mechanically working the sintered product athigh temperatures, such as by forging, swaging and wire drawing,converts the crystalline structure into a fibrous elongated grainstructure. The graingrowth-promoting additives produce exaggerated graingrowth in the central core compared with tungsten crystals notcontaining such additives. On the other hand, the presence of increasingamounts of thoria particles in the surface portion of the dispersionalloy product is effective in restraining grain growth of the tungstencrystals and providing smaller size tungsten crystals at this locationwhen certain thermal treatments are given the material. Such a tungstengrain structure at the core of the wire is effective in minimizing sagwhich is primarily a creep process as well as the further problem ofoffsetting which is due to slip in grain boundaries forming large angleswith the longitudinal working direction. The composite crystallinestructure of the tungsten matrix in the final product should also beobtainable although possibly to a lesser degree in a dispersion alloyproduct containing other types of additives.

In a different preferred embodiment of the present invention, it ispossible to obtain a more creep or sag resistant structure whenrecrystallization takes place by achieving a ThO concentration profilesuch that there is a decreasing concentration of ThO from the surface tothe interior of the product of at least percent. More particularly,since the recrystallized tungsten grain size and recrystallizationtemperature for the dispersion alloy product are influenced by thethoria content, it becomes readily apparent that a high degree ofcontrol can be exercised over the final crystalline structure withmodifications of the diffusion technique. By using differentcombinations of the presintered ingot porosity, the diffusion time, thesolution concentration of the thorium compound, and the concentration ofthe grain-growth-promoting additives in the original compact, it becomesthereby possible to vary the tungsten crystal size uponrecrystallization in the final product from a relatively large grainsize internal core with a surrounding shell of small grain size to amore uniform large grain size throughout the cross section. Therecrystallized tungsten grain size and distribution are also dependentupon the heat treatment schedule employed to obtain the final product.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a graph depictingconcentration gradients of ThO in three different 14 mil wires preparedin accordance with the present invention. These data were obtained usinga point-by-point analysis by the electron beam microprobe technique.These curves are most useful in illustrating the existence of gradientsas well as the difference between the concentration gradients in threedifferent preparations.

FIG. 2 is a photomicrograph ofa longitudinal section of one type of l4mil wire obtained by practice of the present invention. Magnification isat 100 times.

FIG. 3 illustrates the effect of heating rate on the recrystallizedstructure of another type of 14 mil wire product obtained by practice ofthe present invention. Magnification is at 100 times.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Basically, the liquid diffusionprocess practiced in the present invention comprises soaking a porouscompact of tungsten having uniformly dispersed thereingraingrowth-promoting additives disclosed in US. Pat No. 1,410,499 withan inert liquid solvent of a soluble thorium compound until the porestructure of said compact is essentially filled with the solvent andthereafter immersing the solvent-filled compact in a liquid solution ofthe thorium compound for a sufficient time period to permit liquiddiffusion of the dissolved thorium compound into at least a portion ofthe solvent contained in the pores of the compact. The compact is thenremoved from the solution after a predetermined amount of liquiddiffusion has taken place and all solvent is removed to retain aconcentration gradient of the thorium compound in the pores of thecomposite formed. By inert liquid solventis meant a solvent for thethorium compound which can be removed by drying or heating theliquid-filled compact without leaving a residue in the pore structure orundergoing any significant chemical reaction with the tungsten material.In modifications of the above-described liquid diffusion process, therecan be added a second soluble additive in the same liquid solvent forthe thorium compound or a different solvent can be used to fill the porestructure than is employed to dissolve one or more additives. When afirst liquid solvent is used to saturate the compact while a secondliquid solvent is used to prepare the additive solution, it follows thatthe concentration gradient of additive in the porous compact afterremoval of solvent can be controlled by liquid diffusion principlesapplicable to diffusion of the additive from said second liquid solventinto the first liquid solvent.

Specific examples describing preparation of dispersion alloyproductsaccording to the present invention will now be given.

EXAMPLE A A pressed compact measuring 17 mm X 22 mm in cross section wasprepared having a tungsten composition which contained small residualamounts of Al, K and Si in accordance with the teachings of theaforementioned U.S. Pat. No. 1,410,499. The presintered ingot weighingapproximately 1.7 kg was soaked in distilled water until theinterconnected pore structure of the compact was saturated and theliquid-filled ingot thereafter immersed in an aqueous Th(NO solutionhaving a concentration of 260 grams ThO per liter of solution. Theliquid diffusion process was interrupted after approximately 4 hours byremoving the compact from the thorium nitrate solution. The compact wasnext dried slowly in a vacuum oven at approximately 400F, which removedall solvent from the pores of the compact while retaining aconcentration gradient of the thorium additive as specified in Table Ibelow. The thorium additive-containing compact was next subjected to aconventional presintering operation at approximately l,200C in hydrogenwhich converted the thorium nitrate in the pores to thorium oxide(thoria) and thereafter sintered at approximately 2,400C in hydrogen forapproximately 2 hours to produce a densified thoriated tungstencomposition in accordance with the present invention having a density ofapproximately percent of the theoretical value.

Test specimens were obtained from sintered products produced inaccordance with this example as well as in following Examples B and C inorder to measure the thoria concentration gradient and observe thecrystalline structures. The test specimens were examined by a knownautoradiograph technique and all found to have a visible concentrationgradient of the thoria additive with maximum concentration being locatedat the exterior-most surface portion of the sintered compact. All thoriaconcentrations continuously diminished with increasing distance to thecenter portion of the compact along radii extending from its surface tothe axis. Further comparisons of W-ThO materials made via the liquiddiffusion process with conventionally made W-ThO materials were made.The thoria particles in the thoriated tungsten dispersion alloy productsproduced in accordance with the present invention were found to beuniformly smaller in size range without any significant agglomeration ofthoria particles that was found when the products were produced frompractice of conventional powder metallurgy techniques.

Table I summarizes some data pertaining to ingots prepared in accordancewith Examples A, B and C of the present invention. These data wereobtained using standard X-ray fluorescence techniques. Corner and axisvalues were obtained from small -inch cube sampics taken at theselocations. The bulk value was obtained from a A-inch cross-sectionalslice.

It can be noted from Table I that a concentration gradient of thoriaexists which decreases from surface to center such that in each casethere is at least percent by weight thoria at the central axis than atthe surface of each sintered compact.

The sintered compact of Example A was rolled, swaged and drawn to 14.6mil wire in accordance with conventional tungsten working practice. Theapproximate thoria concentration gradient in the prepared wire isreported in the accompanying FIG. 1 graph for all compositions ofExamples A-C as determined by conventional microprobe measurement.Again, it can be noted from FIG. 1 that in each instance from thesurface to the longitudinal axis of the wire there is at least a 10percent less thoria concentration at the central axis than at the wiresurface.

The Example A wire was also subjected to recrystallization by passing anelectric current through the wire in a hydrogen atmosphere. Moreparticularly, the wire was subjected for a 6-minute time period toapproximately 70 percent of the fusion amperage which produced a largetungsten grain structure exhibiting sag or high temperature creepresistance far superior to either pure tungsten or conventionalthoriated tungsten wire of the same size. Upon subjecting a differentspecimen of the Example A wire to recrystallization at l minute up to 80percent ofthe fusion amperage and thereafter maintaining the sameelectric current for an additional 6-minute time period, there wasobtained in some cases a tungsten grainstructure as shown in thephotomicro graph of FIG. 2. Observation of the photomicrographillustrates a fine grain tungsten structure at the surface portion ofthe wire with a large grain structure of tungsten in the central coreregion. In addition to having applicability as a configurationallystable electron source at high temperatures, this structure shouldexhibit improved performance as an incandescent filament in an electriclamp. The small surface grain structure allows the filament in a lamp toabsorb shock and vibration at high temperatures, while the larger graininterior structure provides resistance to filament sag.

EXAMPLE B A presintered compact having the same initial tungstencomposition disclosed in Example A and containing the same solvent wasimpregnated by liquid diffusion with the same aqueous thorium nitratesolution for a time period of approximately 30 hours. Thethoriumcontaining compact was also given the same heat treatmentdisclosed in Example A. A comparison of the values reported in Table Iabove for the dispersion alloy products obtained upon sintering theExamples A and B finds the longer diffusion time to effectively increasethe thoria concentration both at the surface and central axis of thecompact. The sintered compact was next processed to 14.6 mil wire asdescribed in Example A,

and the approximate thoria concentration profile of the wire products isshown in FIG. 1. I

A comparison was made of recrystallization behavior for the wireprepared in this example compared with a control sample of the same sizewire prepared from unthoriated tungsten but containing the sameconcentration of grain-growth-promoting additives. More particularly,the samples were heated by electric current in hydrogen for one minuteup to 70 percent or to 80 percent of the fusion amperage and thereafterheld for an additional 6 minutes at the same heating current. A largegrain tungsten structure was produced in the Example B wire (FIG. 3a)exhibiting excellent sag resistance comparable to the control wirecontaining no thoria for both the 70 and 80 percent current settings. Inadditional experiments, the 6-minute recrystallization temperature ofthe Example B wire was found to be significantly higher by approximately300C than that obtained with the control wire. If the heating time to 70or 80 percent fusion amperage is very short (approximately 1 second), asmall grain structure seen in FIG. 3b can be obtained. That such achange in heating rate could result in the significant change in grainsize noted in FIG. 3 and that the large grain tungsten structure wasobtained in the presence of small ThO particles comprising about 0.8percent by weight of the alloy was not expected.

EXAMPLE C A presintered compact of tungsten containing the sameconcentration of grain-growth-promoting additives was prepared asdescribed in the previous examples. The compact was immersed indistilled water, and the solvent soaked ingot then placed in an aqueousthorium nitrate solution having a concentration of approximately 520grams thoria per liter of solution. The liquid diffusion time for thisexample was 4 hours which provided a thoria concentration gradient inthe sintered ingot as reported in Table I above. The sintered compactwas processed into 14.6 mil wire in the same manner described in thepreceding examples. The approximate thoria concentration gradient in thewire is reported in FIG. 1.

The recrystallization behavior of the Example C wire was quite similarto that for Example B wire under the same electrical heating conditions.Specifically, for a one-minute rise time to or percent fusion amperagein hydrogen, the Example C wire recrystallizes to a large grain tungstenstructure which is sag-resistant. The temperature at which this occursfor 6-minute isochronal anneals is approximately 400C above that for acontrol wire of the same size containing only the same concentration ofthe grain-growth-promoting additives but containing no thoria. Very fastheating rates cause a small grain size to develop analogous to thatshown in FIG. 3b. It will be apparent from all of the foregoingrecrystallization results that a desirably higher recrystallizationtemperature can be obtained by practice of the present invention whilestill achieving desired sag resistance from a relatively large tungstengrain size in the recrystallized product.

It can also be appreciated from the foregoing description of exemplaryembodiments employed to form the novel dispersion alloy products of thepresent invention that various modifications in the liquid diffusiontechnique and the products obtained therefrom can provide even furtherimproved results. For example, still other soluble additives can beincorporated with a thorium compound in a common solvent to permitco-diffusion of the additives. Consequently, it is intended to limit thepresent invention only to the scope of the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A dispersion alloy product of tungsten containinggrain-growth-promoting additives and thoria with the maximumconcentration of thoria being located at the exterior surface portion ofsaid dispersion alloy product and with said thoria concentrationcontinuously and uniformly diminishing with increasing distance to theinterior portion of the dispersion alloy product, thereby establishing aconcentration gradient of thoria having no sharp compositional variationalong said gradient.

2. A product as in claim 1 wherein the growthpromoting additivescomprise potassium, silicon and aluminum compounds.

3. A product as in claim 1 wherein the thoria concentration is at leastpercent less at the center of the product than at the exterior surface.

4. A filament as defined in claim 1.

5. An electric lamp including an incandescible filament as defined inclaim 4.

6. A recrystallized dispersion alloy product of tungsten containinggrain-growth-promoting additives and thoria with a central core ofsag-resistant tungsten crystal grains and a surface portion ofvibration-damping tungsten crystals having a smaller average size thanthe tungsten crystals in the central core, said thoria being dispersedin the dispersion alloy product with a maximum concentration beinglocated at the exterior surface portion of the dispersion alloy productand with said thoria concentration continuously and uniformlydiminishing with increasing distance to the interior portion of thedispersion alloy product thereby establishing a concentration gradientof thoria having no sharp compositional variation along said gradient.

7. A product as in claim 6 wherein the growthpromoting additivescomprise potassium, silicon and aluminum compounds.

8. A product as in claim 6 wherein the thoria concentration is at least10 percent less at the center of the product than at the exteriorsurface.

9. A filament as defined in claim 6.

10. An electric lamp including an incandescible filament as defined inclaim 9.

11. A dispersion alloy product of tungsten containinggrain-growth-promoting additives and thoria with said thoriaconcentration continuously and uniformly di minishing with increasingdistance to the interior portion of the porous body thereby establishinga concen tration gradient of thoria having no sharp compositionalvariation along said gradient, said dispersion alloy product having arecrystallized tungsten grain structure with the size and shape of theindividual tungsten grains being dependent upon the rate of reheatingthe dispersion alloy product after said product has been mechanicallyworked at elevated temperatures.

12. A product as in claim 11 wherein the growthpromoting additivescomprise potassium,'silicon and aluminum compounds.

13. A product as in claim 11 wherein the ThO concentration is at least10 percent less at the center of the product than at the exteriorsurface.

14. A filament as defined in claim 11.

15. An electric lamp including an incandescible filament as defined inclaim 13.

16. A porous metal body having an interconnected pore structure oftungsten containing grain-growthpromoting additives which includesthoria particles dispersed in at least a portion of the pores such thata concentration gradient is established with the maximum thoriaconcentration being located at the exterior sur face portion of saidporous body and with said thoria concentration continuously anduniformly diminishing with increasing distance to the interior portionof the porous body so that no sharp compositional variation exist alongsaid concentration gradient.

17. A porous metal body as in claim 16 wherein the growth-promotingadditives comprise potassium, silicon and aluminum compounds.

18. A porous metal body as in claim 16 wherein the thoria concentrationis at least 10 percent less at the center of the product than at theexterior surface.

2. A product as in claim 1 wherein the growth-promoting additivescomprise potassium, silicon and aluminum compounds.
 3. A product as inclaim 1 wherein the thoria concentration is at least 10 percent less atthe center of the product than at the exterior surface.
 4. A filament asdefined in claim
 1. 5. An electric lamp including an incandesciblefilament as defined in claim
 4. 6. A recrystallized dispersion alloyproduct of tungsten containing grain-growth-promoting additives andthoria with a central core of sag-resistant tungsten crystal grains anda surface portion of vibration-damping tungsten crystals having asmaller average size than the tungsten crystals in the central core,said thoria being dispersed in the dispersion alloy product with amaximum concentration being located at the exterior surface portion ofthe dispersion alloy product and with said thoria concentrationcontinuously and uniformly diminishing with increasing dIstance to theinterior portion of the dispersion alloy product thereby establishing aconcentration gradient of thoria having no sharp compositional variationalong said gradient.
 7. A product as in claim 6 wherein thegrowth-promoting additives comprise potassium, silicon and aluminumcompounds.
 8. A product as in claim 6 wherein the thoria concentrationis at least 10 percent less at the center of the product than at theexterior surface.
 9. A filament as defined in claim
 6. 10. An electriclamp including an incandescible filament as defined in claim
 9. 11. Adispersion alloy product of tungsten containing grain-growth-promotingadditives and thoria with said thoria concentration continuously anduniformly diminishing with increasing distance to the interior portionof the porous body thereby establishing a concentration gradient ofthoria having no sharp compositional variation along said gradient, saiddispersion alloy product having a recrystallized tungsten grainstructure with the size and shape of the individual tungsten grainsbeing dependent upon the rate of reheating the dispersion alloy productafter said product has been mechanically worked at elevatedtemperatures.
 12. A product as in claim 11 wherein the growth-promotingadditives comprise potassium, silicon and aluminum compounds.
 13. Aproduct as in claim 11 wherein the ThO2 concentration is at least 10percent less at the center of the product than at the exterior surface.14. A filament as defined in claim
 11. 15. An electric lamp including anincandescible filament as defined in claim
 13. 16. A porous metal bodyhaving an interconnected pore structure of tungsten containinggrain-growth-promoting additives which includes thoria particlesdispersed in at least a portion of the pores such that a concentrationgradient is established with the maximum thoria concentration beinglocated at the exterior surface portion of said porous body and withsaid thoria concentration continuously and uniformly diminishing withincreasing distance to the interior portion of the porous body so thatno sharp compositional variation exist along said concentrationgradient.
 17. A porous metal body as in claim 16 wherein thegrowth-promoting additives comprise potassium, silicon and aluminumcompounds.
 18. A porous metal body as in claim 16 wherein the thoriaconcentration is at least 10 percent less at the center of the productthan at the exterior surface.